Device and method for using a magnetic clutch in BLDC motors

ABSTRACT

An apparatus for coupling mechanical power between the rotor of a Brushless DC Motor and an external mechanical load comprises: a) two concentric rings; b) an equal number of magnets connected to the inner ring and to the outer ring; and c) an opposite orientation of the poles of each couple of facing magnets, wherein one magnet is placed on the inner ring, and its facing magnet is placed on the outer ring; wherein the first of said two concentric rings is rotatable around an axis by the application of a force not applied by the second ring, and wherein when said first concentric ring rotates, the second ring rotates as well by the action of magnetic forces.

PRIORITY

This application is a Continuation of U.S. application Ser. No.14/772,005, filed Sep. 1, 2015, which is a U.S. national phase entry ofPCT/IL2014/050286, filed Mar. 13, 2014, which claims priority toPCT/IL2013/050253, filed Mar. 19, 2013, which claims priority to IL218743, filed Mar. 20, 2012, the full disclosures of which areincorporated herein in their entireties by reference.

FIELD OF THE INVENTION

The present invention relates to a magnetic clutch architecture designedto couple mechanical power between the rotor of Brushless DC Motors(BLDC) and an external mechanical load, without using direct or indirectmechanical connection such as gears, wheels, strips or other similararrangements.

BACKGROUND OF THE INVENTION

In many common systems, the connection between different parts of thesystem is performed by mechanical components. A significant disadvantageof using such connecting parts is the energy loss, caused by friction.Another disadvantage caused by friction is the wear of the connectingsurfaces of the parts. As the speed and force between the partsincrease, so does the friction and therefore the damage to theirsurfaces, until they often can no longer function properly.

In systems operating at high speeds, like motors that usually operate inextremely high speeds, the friction and its outcomes are substantial,resulting in the need for many maintenance services and frequent changeof parts, which require a great investment of both time and money.

The present invention relates to a device used in BLDC motors, such asthe motor described in PCT patent application No. PCT/IL2013/050253

It is an object of the present invention to provide a device and methodthat overcome the drawbacks of the prior art.

Other objects and advantages of the invention will become apparent asthe description proceeds.

SUMMARY OF THE INVENTION

An apparatus for coupling mechanical power between the rotor of aBrushless DC Motor and an external mechanical load, comprises:

a) two concentric rings;

b) an equal number of magnets connected to the inner ring and to theouter ring; and

c) an opposite orientation of the poles of each couple of facingmagnets, wherein one magnet is placed on the inner ring, and its facingmagnet is placed on the outer ring;

-   -   wherein the first of said two concentric rings is rotatable        around an axis by the application of a force not applied by the        second ring, and wherein when said first concentric ring        rotates, the second ring rotates as well by the action of        magnetic forces.

In one embodiment of the invention the rings are flat ring-shapedplates. In another embodiment of the invention each couple of facingmagnets are of the same size.

In some embodiments of the invention the magnetic strengths of twofacing magnets are essentially the same. In another embodiment of theinvention each of the magnets in the inner ring has a facing magnet inthe outer ring.

The connecting means, in some embodiments of the invention, connect oneof the rings to an external system. In other embodiments of theinvention a ring which is not connected to the external system is drivenby the rotation of the ring that is connected to the external system andthe driven ring is forced to move because of the magnetic force betweentwo coupled magnets.

Typically, the distances between the components of the apparatus areconsistent with the desired forces and in some embodiments of theinvention the distance between two adjacent magnets on the ring is notthe same as the distance between two other adjacent magnets on the samering.

The invention also encompasses a brushless motor coupled with a clutchcomprising two concentric rings, an equal number of magnets connected tothe inner ring and to the outer ring, and an opposite orientation of thepoles of each couple of facing magnets, wherein one magnet is placed onthe inner ring, and its facing magnet is placed on the outer ring, andwherein the first of said two concentric rings is rotatable around anaxis by the application of a force not applied by the second ring, andwherein when said first concentric ring rotates, the second ring rotatesas well by the action of magnetic forces.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows two concentric rings, provided with magnets, according toone embodiment of the invention, in a static state;

FIG. 2 shows the two rings of FIG. 1 in a dynamic state;

FIG. 3 shows the measurements of the force on a single couple of magnetsmounted at distance d from each other and shifted linearly;

FIG. 4 shows the measurements of the force in a demo system, accordingto another embodiment of the invention;

FIG. 5 shows exemplary physical measures of the components in a BLDCdemo system, according to another embodiment of the invention;

FIG. 6 shows a schematic setup of two magnets, according to anotherembodiment of the invention;

FIG. 7 shows solenoids illustrated as consisting of a collection ofinfinitesimal current loops, stacked one on top of the other; and

FIG. 8 shows two loops of infinitesimal thickness, each one belonging toa magnet.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows two concentric rotating rings 101 and 102 at rest. Theinner ring 101 consists of the rotor of a BLDC motor (which can be, forexample, the motor of PCT/IL2013/050253-WO/2013/140400), and the outerring 102 is connected to a mechanical load and provides the power forit. A number of permanent magnets, equal to the number of the magnets inthe rotor of the BLDC motor, are mechanically fixed on the outer ring102 with their S-N axes oriented tangentially to the circumference.

At rest, each one of the magnets 104 located on the outer ring 102, isfacing the corresponding magnet 103 located on the rotor 101. The S-Naxis orientation of each magnet 104 on the outer ring 102 is opposite tothe S-N axis orientation of the corresponding (facing) magnet 103 on therotor 101. As a result, the magnets 104 on the outer ring 102 arepositioned with alternating polarity. Each pair of magnets 104 on outerring 102 may be equidistantly spaced from one another by acircumferential distance of F. Alternatively, one pair of magnets 104 onouter ring 102 may be spaced from one another by a circumferentialdistance of F, while another pair of magnets 104 on the same may bespaced from one another by a circumferential distance of G which may begreater than F. It should be emphasized that there is no physicalconnection between the rotor 101 and the outer ring 102. For reasonsthat will be thoroughly explained later on in this description, based onthe laws of magnetostatics, the relative position of the rotor 101 withrespect to the outer ring 102, depends on the state of the system—if thesystem is in a static state or a dynamic state, as will be furtherdescribed.

In a static state—when the BLCD rotor is at rest, each magnet 104 on theouter ring 102 is exactly aligned in front of the corresponding magnet103 on the rotor 101, as shown in FIG. 1. In a dynamic state—when theBLCD rotor 101 turns, while the outer ring 102 is connected to a load(not completely free to move), the relative position of each magnet 103on the rotor ring 101 with respect to the corresponding magnet 104 onthe load ring 102, will change and will stabilize to a new state.

The corresponding magnets 103 and 104 will no longer be perfectlyaligned. The relative position of the magnets will shift in aquasi-linear fashion tangentially to the circumference of the rings 101and 102. The magnets 103 and 104 will reach an offset h as shown in FIG.2, and will stabilize there. The offset h will depend on the opposingforce exercised by the load. It will be seen that under properconditions h will increase directly proportionally to the force neededto make the load ring 102 rotate along with the rotor ring 101.

It will be presented that in the range of interest, the offset h isroughly directly proportional to the force transfer, and as long as h isnot too large, the rotor ring 101 will be able to “pull along” the loadring 102, without the occurrence of any physical contact between the tworing 101 and 102. When the size of h approaches the width of the gapbetween the magnets 103 and 104, the force transferred drops. Themaximal force that the rotor ring 101 will be able to apply to the loadring 102 will depend on the strength and on the geometry of thepermanent magnets, on the number of magnets, as well as on the gapbetween the two rings 101 and 102.

FIG. 3 shows the measurements of the force on a single couple of magnetsmounted at distance d from each other and shifted linearly. The shadedarea 301 shows the range for which the pulling force between the magnets103 and 104 is roughly proportional to the offset h.

To illustrate the order of magnitude of the forces involved, two magnetswith front-to-front separation of 29 mm, can provide roughly a maximalforce transfer of 140N (about 14 Kg) in direction tangential to thering.

In the BLDC motor demo system built according to one embodiment of thepresent invention, there are 8 magnets were provided with face-to-faceseparation of about 30 mm. The demo system is capable to apply a forceof 140×8=1120N (about 112 Kg). Since the outer ring 102 in the demosystem has a radius of about 420 mm, the magnetic clutch should be ableto transfer a torque of about 470N-m.

In a measurement carried out on the BLDC demo system, and as shown inFIG. 4, the inventors did not try to achieve and measure the maximalpower transfer, however, they showed force transfer measurements of theorder of 600N, which is in good agreement with the order of magnitude ofthe maximal possible force (1120N) predicted by the measurements on onecouple of magnets. Also it shows that the total force is proportional tothe relative offset.

The physical measures of the components in the BLDC demo system,according to some embodiments as provided by the inventors, are shown inFIG. 5. From the figure one can see that the system includes 8 magnets,and the separation between the rotor ring 101 and the load ring 102 is30 mm. In some embodiments, the separation distance may be greater than18 mm. In some embodiments, the separation distance may be less than 35mm.

A plurality of circumferentially spaced and stationary air-coresolenoids 132 constituting a stator of the Brushless DC Motor 100encircle both the inner edge 107 and the outer edge 109 of inner ring101. When inner ring 101 is rotated, the circumferentially spacedpermanent magnets 103 fixed to inner ring 101 are allowed to passthrough the interior 134 of each air-core solenoid 132. The rotation ofinner ring 101 is made possible by the operations of schematicallyillustrated actuator 124, which may be embodied by a plurality ofswitches controlled by a microcontroller for determining, at eachinstant, the polarity and magnitude of voltage applied to each solenoid,as well as the average DC level.

During operation of actuator 124, the air-core solenoids 132 becomeenergized and generate an electromagnetic field that produces a torqueon a magnetic field of the magnet 103 passing through a given solenoidinterior 134, causing inner ring 101 to rotate around its axis 108. Asinner ring 101 rotates, outer ring 102 rotates as well under theinfluence of magnetic forces between each couple of magnetically facingmagnets 102 and 103 of opposite magnetic polarity. Connecting means mayconnect the rotating outer ring 102 to a load 126. The connecting meansmay be, for example, a plurality of circumferentially spaced linearelements 137 that are connected to outer ring 102 and radially extendtherefrom to a mechanical load 126 which is located at center of theinner 101 and outer rings 102.

Magnetostatic computations are among the most difficult and complextasks to be carried out analytically, and even when a closed-formanalytical expression can be found, the resulting formulas are often toocomplex to provide a clear understanding of the phenomena. Moreover,most often, one can only perform computerized simulations obtained bynumerically solving the field equations. Numerical solutions, however,although precise for a specific setup, do not provide an insight to thegeneral behavior of the system.

Fortunately, in the specific case under consideration, generalconclusions can be drawn by means of a relatively simple mathematicalanalysis. This is made possible because, in the system underconsideration, the magnets are free to move only along a directiontangential to their S-N axis, and they are fixed in all otherdirections. Therefore, it is only needed to compute the component of theforce in a direction parallel to the S-N axes of the magnets, whichresults in major mathematical simplifications that allow us to drawconclusion regarding general system features, without the need ofactually solving the complex three-dimensional integrals involved.

What was analyzed is the setup shown in FIG. 6. {circumflex over (x)}, ŷand {circumflex over (z)} are mutually perpendicular unit vectors. Twocubic magnets 601 and 602 are positioned so that their S-N axes areparallel to direction {circumflex over (z)}. Their S-N orientation isopposite, and they are displaced with an offset h in direction{circumflex over (z)}. The magnets 601 and 602 are assumed cubic, forthe purpose of this exemplary analysis, however the general conclusionshold true for other shapes as well. The measurements shown in FIG. 3have been carried out on a similar setup.

Under this setup, as long as the offset h is small relatively to thephysical dimension of the gap between the magnets 601 and 602, thecomponent of the force acting on either magnet 601 and 602 in thedirection {circumflex over (z)}, is directly proportional to the offseth. The size of h is relatively small, roughly when the offset h is lessthan ⅓ of the separation d between the magnets 601 and 602. As theoffset becomes larger than that, the force reaches a maximal value, andthen decreases with increasing h.

As a first step, by using the Amperian model, a permanent magnet withmagnetization M in direction {circumflex over (z)}, may be modeled inthe form of a uniform surface current density

, flowing on the surface of the magnet in direction perpendicular to{circumflex over (z)}.

is the net magnetic dipole moment per unit volume, and

is the equivalent surface current per unit length. Therefore we mayreplace each magnet 601 and 601 in FIG. 6 by the equivalent “solenoids”shown in FIG. 7, with equal currents in opposite directions.

Each solenoid 701 in FIG. 7 can be represented as consisting of acollection of infinitesimal current loops, stacked one on top of theother, carrying currents of amplitudes dl=J_(s)dz and dl′=J_(s)dz′,flowing in the plane in {circumflex over (x)}ŷ plane in oppositedirections. Let us consider now, two loops of infinitesimal thickness,each one belonging to one of the magnets as shown in FIG. 8.

The force caused on the left-side loop L located at vertical position zthe right-side loop L′ located at vertical position z′, is directlyderived from Ampere's law of force, and is given by the expression:

${{\overset{\rightarrow}{F}}_{p^{\prime}p}\left( {z,z^{\prime}} \right)} = {{{- \frac{\mu\;{dI}^{\prime}{dI}}{4\pi}}{\int_{L}{\int_{L^{\prime}}{\left( {d\;{\hat{\ell} \cdot d}\;{\hat{\ell}}^{\prime}} \right)\frac{{\hat{r}}_{p^{\prime}p}}{{{\hat{r} - {\hat{r}}^{\prime}}}^{2}}}}}} = {{- \frac{\mu\;{dI}^{\prime}{dI}}{4\pi}}{\int_{L}{\int_{L^{\prime}}{\left( {d\;{\hat{\ell} \cdot d}\;\hat{\ell}} \right)\frac{\hat{r} - {\hat{r}}^{\prime}}{{{\hat{r} - {\hat{r}}^{\prime}}}^{3}}}}}}}$${{{where}\mspace{14mu}{\hat{r}}_{p^{\prime}p}} = \frac{\hat{r} - {\hat{r}}^{\prime}}{{r - {\hat{r}}^{\prime}}}},{{\hat{r} - {\hat{r}}^{\prime}} = {{\left( {x - x^{\prime}} \right)\hat{x}} + {\left( {y - y^{\prime}} \right)\hat{y}} + {\left( {z - z^{\prime}} \right)\hat{z}}}},{{{\hat{r} - {\hat{r}}^{\prime}}} = \sqrt{\left( {x - x^{\prime}} \right)^{2} + \left( {y - y^{\prime}} \right)^{2} + \left( {z - z^{\prime}} \right)^{2\;}}}$And d{circumflex over (l)} and d{circumflex over (l)}′ are infinitesimallengths in the direction of the current flow in the corresponding loops,and therefore they lie in the {circumflex over (x)}ŷ plane.

Now, referring to FIG. 8, it points out several preliminary remarks:

1. We know that |y−y′|≥d and we denoteR_({circumflex over (x)}ŷ)≡√{square root over ((x−x′)²+(y−y′)²)}. Itfollows that R_({circumflex over (x)}ŷ)≥d.R_({circumflex over (x)}ŷ)(x,x′,y,y′) is independent from z and z′, andwe may write |{circumflex over (r)}−{circumflex over(r)}′|=R_({circumflex over (x)}ŷ) ²+(z−z′)².

2. In the present setting, d is comparable to the size of the magnet,and we assume offsets small enough so that

²<<d²

$\left( {{{for}\mspace{14mu}{instance}\mspace{14mu}} < \frac{d}{3}} \right).$

3. Since we are interested only in the force in the {circumflex over(z)} direction, the only relevant component of {circumflex over(r)}-{circumflex over (r)}′ in the numerator of the integrand, is theone in direction {circumflex over (z)}. All other forces are of nointerest, since the magnets cannot move in other directions. Thus, inorder to compute the force acting on the magnets in {circumflex over(z)} direction, we may replace {circumflex over (r)}-{circumflex over(r)}′ in the numerator of the integrand by (z−z′){circumflex over (z)}.

4. d{circumflex over (l)} and d{circumflex over (l)}′ are incrementalvectors in the {circumflex over (x)}ŷ plane. More precisely, in thepresent setting of square magnets, the scalar product (d{circumflex over(l)}·d{circumflex over (l)}′) is either ±dxdx′ or ±dydy′. Therefore zand z′ are constant with respect to the integration variables whenintegrating over the path of the loops. Moreover, if dx, dx′ haveopposite signs, their direction of integration is opposite too, andtherefore, the limit of the corresponding integrals are reversed, andsimilarly for dy, dy′. The outcome is that the sign of the integral forall the various sub-integration ranges defined by (d{circumflex over(l)}·d{circumflex over (l)}′) remains unchanged. Therefore the signvalue of the double integral over the loop paths, is the same as thesign of the integrand.

With the above understanding, the force ΔF_(z) in the direction{circumflex over (z)} acting on the current loop L because of thecurrent loop L′, is the result of the following integral:

${{\Delta\; F_{\hat{z}}} = {\frac{\mu\; J_{s}^{2}{dz}^{\prime}{dz}}{4\pi}{\int_{L}{\int_{L^{\prime}}\frac{\left( {d\;{\hat{\ell} \cdot d}\;{\hat{\ell}}^{\prime}} \right)\left( {z - z^{\prime}} \right)}{\left\lbrack {R_{\hat{x}\hat{y}}^{2} + \left( {z - z^{\prime}} \right)^{2}} \right\rbrack^{3/2}}}}}},{R_{\hat{x}\hat{y}}\sqrt{\left( {xx}^{\prime} \right)^{2} \cdot \left( {yy}^{\prime} \right)^{2}}},{{dI}^{\prime}J_{\Delta}{{dz}^{\prime} \cdot {dIJ}_{\Delta}}{dz}}$

The cumulative force ΔF_({circumflex over (z)},L) applied by all thecurrent loops on the right side on one single current loop L on the leftside (see FIG. 8) is given by:

${\Delta\; F_{\hat{z},L}} = {{\int_{h}^{h + a}{\Delta\; F_{\hat{z}}{dz}^{\prime}}} = {{- \frac{\mu\; J_{s}^{2}{dz}}{4\pi}}{\int_{h}^{h + a}{\left( {\int_{L}{\int_{L^{\prime}}\frac{\left( {d\;{\hat{\ell} \cdot d}\;{\hat{\ell}}^{\prime}} \right)\left( {z - z^{\prime}} \right)}{\left\lbrack {R_{\hat{x}\hat{y}}^{2} + \left( {z - z^{\prime}} \right)^{2}} \right\rbrack^{3/2}}}} \right){dz}^{\prime}}}}}$

The total force F_({circumflex over (z)})(h) acting on the magnetlocated at the origin is the sum of all the forces on its loops:

${F_{\hat{z}}(h)} = {{\int_{0}^{a}{\Delta\; F_{\hat{z},L}{dz}}} = {{- \frac{\mu\; J_{s}^{2}}{4\pi}}{\int_{0}^{a}{\left\lbrack {\int_{h}^{h + a}{\left( {\int_{L}{\int_{L^{\prime}}\frac{\left( {d\;{\hat{\ell} \cdot d}\;{\hat{\ell}}^{\prime}} \right)\left( {z - z^{\prime}} \right)}{\left\lbrack {R_{\hat{x}\hat{y}}^{2} + \left( {z - z^{\prime}} \right)^{2}} \right\rbrack^{3/2}}}} \right){dz}^{\prime}}} \right\rbrack{dz}}}}}$

Changing the order of integration we obtain:

${F_{\hat{z}}(h)} = {{- \frac{\mu\; J_{s}^{2}}{4\pi}}{\int_{L}{\int_{L^{\prime}}{\left\lbrack {\int_{0}^{a}{\left( {\int_{h}^{h + a}{\frac{\left( {z - z^{\prime}} \right)}{\left\lbrack {R_{\hat{x}\hat{y}}^{2} + \left( {z - z^{\prime}} \right)^{2}} \right\rbrack^{3/2}}{dz}^{\prime}}} \right){dz}}} \right\rbrack\left( {d\;{\hat{\ell} \cdot d}\;{\hat{\ell}}^{\prime}} \right)}}}}$Noting that R_({circumflex over (x)}ŷ) ² is independent from z and z′,and therefore is constant when integrating with respect to dz and dz′,the inner integrals can be computed analytically, and yield:

${\int_{0}^{a}{\left( {\int_{h}^{h + a}{\frac{\left( {z - z^{\prime}} \right)}{\left\lbrack {R_{\hat{x}\hat{y}}^{2} + \left( {z - z^{\prime}} \right)^{2}} \right\rbrack^{3/2}}{dz}^{\prime}}} \right){dz}}} = {\ln\left\{ \frac{\left\lbrack {\left( {ɛ - A} \right) + \sqrt{1 - \left( {ɛ - A} \right)^{2}}} \right\rbrack\left\lbrack {\left( {ɛ - A} \right) + \sqrt{1 - \left( {ɛ - A} \right)^{2}}} \right\rbrack}{\left( {ɛ + \sqrt{1 + ɛ}} \right)^{2\;}} \right\}}$Where we used

${A = \frac{a}{R_{\hat{x}\hat{y}}}},{{{and}\mspace{14mu} ɛ} = {\frac{h}{R_{\hat{x}\hat{y}}}.}}$

Since R_({circumflex over (x)}ŷ)≥d, then ifh²<<d²≤R_({circumflex over (x)}ŷ) ²

${{\left( {{{for}\mspace{14mu}{instance}\mspace{14mu} h} < \frac{d}{3}} \right)\mspace{14mu}{then}\mspace{14mu}\frac{h^{2}}{R_{\hat{x}\hat{y}}^{2}}} = {ɛ^{2} ⪡ 1}},$and we may expand the last expression in a first-order Taylor series asfollows:

${\int_{0}^{a}{\left( {\int_{h}^{h + a}{\frac{\left( {z - z^{\prime}} \right)}{\left\lbrack {R_{\hat{x}\hat{y}}^{2} + \left( {z - z^{\prime}} \right)^{2}} \right\rbrack^{3/2}}{dz}^{\prime}}} \right){dz}}} = {{{{2\frac{1 - \sqrt{1 + A^{2\;}}}{\sqrt{1 + A^{2}}}ɛ} + {O\left( ɛ^{3} \right)}} \approx {2\;{\frac{1 - \sqrt{1 + {a^{2}/R_{\hat{x}\hat{y}}^{2}}}}{\sqrt{1 + {a^{2}/R_{\hat{x}\hat{y}}^{2}}}} \cdot \frac{h}{R_{\hat{x}\hat{y}}}}}} = {{g\left( {x,x^{\prime},y,y^{\prime}} \right)} \cdot h}}$

Since √{square root over (1+a²/R_({circumflex over (x)}ŷ) ²)}>1, itfollows that the function g(x, x′,y,y′) is some negative function of (x,x′, y, y′), namely g(x, x′,y,y′)=−|g (x, x′,y,y′). Therefore, recallingthat the sign of the double integral over x, x′, y, y′ is the same asthe sign of the integrand, and setting

${{\int_{L}{\int_{L^{\prime}}{{{g\left( {x,x^{\prime},y,y^{\prime}} \right)}}\left( {d\;\hat{\ell}\; d\;{\hat{\ell}}^{\prime}} \right)}}} = {\frac{K^{2}\mu\; J_{s}^{2}}{4\pi}h}},{h^{2} ⪡ {d^{2}.}}$Where K is some proportionality constant. Finally, recalling thatM=J_(s) is the net magnetization per unit volume in the {circumflex over(z)} direction, and referring to FIG. 6, the force acting on the leftmagnet is:

${{F_{\hat{z}}(h)} = {\frac{K^{2}\mu\; M^{2}}{4\pi}h}},{h^{2} ⪡ d^{2}}$

Thus, for any offset h<d/3, the force transferred by the clutch isdirectly proportional to the offset h and to the square magnetizationper unit volume. Moreover, the force is in direction of the offsetitself.

All the above description has been provided for the purpose ofillustration and is not meant to limit the invention in any way. Thecomputations shown above are provided as an aid in understanding theinvention, and should not be construed as intending to limit theinvention in any way.

The invention claimed is:
 1. A brushless motor comprising a) twoconcentric rings consisting of an inner ring constituting a rotor of abrushless DC motor and an outer ring constituting a clutch, each of saidtwo rings having an inner edge and an outer edge; b) an equal number ofcircumferentially spaced magnets connected to the inner ring and to theouter ring, such that each couple of facing magnets are of oppositemagnetic polarity, wherein one magnet is connected to the inner ring,its facing magnet is connected to the outer ring and magneticallycoupled to said one magnet and the distance between the facing magnetsof each couple is no less than 18 mm and no greater than 35 mm; c) aplurality of circumferentially spaced and stationary air-core solenoidsconstituting a stator of said brushless DC motor, each of said air-coresolenoids encircling both said inner edge and said outer edge of theinner ring so that, through an interior of each of said air-coresolenoids, the magnets of the inner ring can pass; and d) an actuatorfor energizing said air-core solenoids to generate an electromagneticfield that produces a torque on a magnetic field of the magnet passingthrough their interior to rotate the inner ring, as well as the outerring, by means of said magnetically coupled magnets.